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Low-Cost and Reliable Packaging Technology for Stacked MCP with MEMS and Control IC Chips

Keywords: MEMS, Wafer Level Package, Stacked MCP

Since MEMS consist of movable parts and need to be operated in controlled atmosphere, the conventional package techniques for LSI cannot be applied directly.
In many cases, as a MEMS chip, which needs to be controlled by another IC chip, is packaged using silicon or glass cap, it has been difficult to reduce the package size and cost.
To address these issues, we have developed the hermetic thin-film encapsulation structure fabricated by conventional BEOL technologies of LSI.
The fabrication process of the thin-film encapsulation is as follows.
First, 6ƒÊm-thick polymer film is coated, cured and patterned as a sacrificial layer on the MEMS element. Next, 3ƒÊm-thick cap layer is formed by PE-CVD, and some holes for removing the sacrificial layer are opened by the conventional photolithography and dry etching. Then, the sacrificial layer is removed through these holes by dry ashing and the holes are sealed by a spin coated polymer. Finally, an inorganic film is stacked by PE-CVD to protect MEMS against moisture penetration, and the contact pads are opened. The thin-film encapsulation structure is only 20ƒÊm-height and protects MEMS movable parts.
In order to stack the MEMS and control IC chips, we have applied the conventional MCP process. The thin-film encapsulation structure allows the MEMS wafer to be ground.
The 100ƒÊm-thick MEMS is stacked on a 65ƒÊm-thick control IC mounted on a BT substrate, followed by epoxy encapsulation and BGA balls mounting process. Then we have obtained 0.8mm-thick MCP, which is the thinnest package for stacked MEMS and control IC chips.
The MEMS actuator was successfully operated in the MCP even at 85% relative humidity, while the MEMS actuator without the thin-film encapsulation caused an irreversible stiction as a fatal failure. Furthermore, the MCP showed excellent durability against 260 degC reflow soldering, unbiased HAST and thermal cycling.